Radioimmunoassay of Digoxin—Interference by Digitoxin

Abstract Digitoxin interference with radioimmunoassay of digoxin may present a problem in determining digoxin concentrations in the serum of patients treated with digitoxin less than four weeks before digoxin was administered. Digoxin and digitoxin values were determined in the sera of 54 patients who were receiving only digitoxin. Additionally, digoxin was added to aliquots of sera from a patient receiving digitoxin, and digoxin concentrations were determined by radioimmunoassay. Digitoxin interfered with radioimmunoassay of digoxin in that 9% of the digitoxin present was counted as digoxin. The resulting spuriously high value for measured digoxin in patients receiving digoxin, but treated with digitoxin previously, can be corrected by applying the following equation (units: µg/liter): Digoxin (corrected) = digoxin (measured) — [0.09 x digitoxin (measured)]. This correction, which may differ somewhat with batch of antiserum, is applicable for digitoxin concentrations up to about 50 ng/ml. Digoxin does not interfere with the digitoxin assay.